Improved air-stability and conductivity in the 75Li2S·25P2S5 solid-state electrolyte system: the role of Li7P3S11

Doping modification is regarded as a simple and effective method for increasing the ionic conductivity and air stability of solid state electrolytes. In this work, a series of (100−x)(0.75Li2S·0.25P2S5)·xP2O5 (mol%) (x = 0, 1, 2, 3 and 4) glass-ceramic electrolytes were synthesized by a two-step ball milling technique. Various characterization techniques (including powder X-ray diffraction, Raman and solid-state nuclear magnetic resonance) have proved that the addition of P2O5 can stimulate 75Li2S·25P2S5 system to generate the high ionic conductivity phase Li7P3S11. Through the doping optimization strategy, 98(0.75Li2S·0.25P2S5)·2P2O5 glass-ceramic (2PO) not only had a 3.6 times higher ionic conductivity than the undoped sample but also had higher air stability. Its ionic conductivity remained in the same order of magnitude after 10 minutes in the air. We further investigated the reasons why 2PO has a relatively high air stability using powder X-ray diffraction and scanning electron microscopy in terms of crystal structure degradation and morphology changes. In comparison to the undoped sample, the high ionic conductivity phases (β-Li3PS4 and Li7P3S11) of 2PO were better preserved, and less impurity and unknown peaks were generated over a short period of exposure time. In addition, the morphology of 2PO only changed slightly after 10 minutes of exposure. Despite the fact that the particles aggregated significantly after several days of exposure, 2PO tended to form a protective layer composed of S8, which might allow some particles to be shielded from attack by moisture, slowing down the decay of material properties.


Supplementary Text
The calculation of ionic conductivity and activation energy of the samples Ionic conductivity (σ) values were calculated by Eq. (S1) using resistance values obtained from the EIS.
where d (cm) is the thickness of solid state electrolytes, S (cm -2 ) belongs to the effective electrolyte area, and R represents the total electrolyte resistance.
In addition, activation energies (Ea) could be calculated using Eq.(S2), where σ0 is the pre-exponential factor, R is the gas constant, and T is the absolute temperature.

Exposure method A for pXRD
Perform pXRD experiments using the silicon bubble holder as shown in the picture to ensure that the sample is in a sealed state during the testing process.First, the sample was prepared in a glove box filled with nitrogen.This sample was called 0 min (corresponding to the mark in Fig. 10).After the test, the bubble holder lid was opened to expose the sample to the air environment, quickly placed it in the glove box chamber with a volume of approximately 30000 cm 3 , and quickly tightened the chamber lid (the entire process took about 10 seconds).The glove box chamber has been opened and filled with air for 1 minute in advance.After 1 minute in the air-filled chamber, the sample was transferred to the glove box and covered.This sample was named 1 min.In the same way, the sample was placed back into the glove box chamber again after testing and left for 1 minute (being exposed to air for a total of 2 minutes).This sample was called 2 min.Following that, the sample was exposed to air for another 1 minute, 2 minutes, 2 minutes, and 3 minutes, and was given the names 3 min, 5 min, 7 min, and 10 min.

Exposure method B for pXRD
In the glove box, 0.1g of sample was placed in each of the four vials with a diameter of 10 mm and a volume of 28 ml.Then, the vials were sealed and removed from the glove box.In the fume hood, the vial lids were opened and the samples were exposed to air for 1 day, 2 days, 3 days and 7 days, respectively.

Exposure method A for SEM
The tested sample was always placed in the SEM chamber with a volume of 38000 cm 3 until the test was completed.The exposure time of the sample in the air was controlled by vacuuming and venting the SEM chamber via the SEM console.Since vacuuming and venting are both gradual processes, timing commenced when they were finally finished.The vacuuming and venting processes took about 3 minutes in total.

Exposure method B for SEM
The sample preparation is the same as Exposure method B for pXRD.
Raman shift / cm

Fig. S1 .Fig
Fig. S1.Deconvoluted Raman spectra of 1PO.The black line represents the experimental data.The blue, green and red lines are the deconvoluted signals attributed to PS 4 3-, P 2 S 6 4-and P 2 S 7 4- moieties, respectively.

Fig
Fig. S4.(a) pXRD patterns of 0PO exposed to air for 1 min, 2 min, 3 min, 5 min, 7 min and 10 min.(b) is an enlarged view of (a) in the 2θ of 25° to 35° range.

Fig
Fig. S7.EDS elemental mapping of 2PO exposed to air for 2 days.

Fig. S8 .
Fig. S8.SEM and EDS images of 2PO exposed to air for 7 days.

Table S4 .
Peak assignments for the 2PO.

Table S5 .
Peak assignments for the 3PO.

Table S6 .
Peak assignments for the 4PO.

Table S7 .
The fitting parameters of deconvoluted Raman spectra of 1PO.FWHM is an abbreviation for full width of the peak at half maximum value.

Table S8 .
The fitting parameters of deconvoluted Raman spectra of 2PO.

Table S11 .
The fitting parameters of deconvoluted 31 P MAS NMR of 2PO.

Table S12 .
The fitting parameters of deconvoluted 31 P MAS NMR of 3PO.

Table S13 .
The fitting parameters of deconvoluted 31 P MAS NMR of 4PO.

Table S14 .
The ionic conductivity of 2PO at various temperatures.